The broad objectives of the proposed research are to chart the acquisition and utilization of hierarchical structure among common categories corresponding to terms in the natural language. A longitudinal study is proposed in which the child's changing comprehension of basic level and superordinate category terms will be followed for two years starting at age one. Two techniques will be used: a) selection of objects in the home environment in response to requests by the mother and b) selection of pictures of single objects, and collections. The object selection technique will map the extension of category terms of different levels over time. The picture selection technique will permit evaluation of alternative hypothesis concerning the child's initial interpretation of superordinate category terms. A second study will examine the power of hierarchies to support induction as a function of age and the nature of the hierarchy. The inductive force of general statements about category inclusion relations and attribute possession will be assessed by asking subjects. a) to evaluate exceptions to general statements ("could you have a dog that's not an animal?" "...a cat without fur?") and b) to describe the nature of acceptable violations. The age range from pre-school (five-year olds) to adults (college students) will be sampled. Psychological and philosophical considerations permit prediction of relative acceptability of violations of different relations at different ages.